1. Technical Field
The present disclosure relates to noise reduction devices used on the insides of closed-structure bodies including aircraft and railway vehicles.
2. Description of the Related Art
Japanese Patent Unexamined Publication No. H07-160280 discloses the method of enhancing an effect of eliminating low-frequency components of a noise, which is applied to silencers for electrical equipment such as air conditioners. The enhancement is achieved by taking consideration of, in the silencers, installation positions of a microphone and speaker, and a delay time between time of noise propagation and time of emitting a control sound from the speaker. Japanese Patent Unexamined Publication No. H10-171468 discloses the method of enhancing an effect of silencing a random noise. The enhancement is achieved by taking consideration of an installation position of a speaker relative to a place (referred to also as “silencing center” or “control point,” hereinafter) where the noise is reduced. Japanese Patent Unexamined Publication No. 2010-188752 discloses the method of effectively exhibiting an effect of reducing a noise even under circumstances that the time causality constraints are not satisfied because of an unfavorable positional relation between a noise-detecting microphone and speaker and the silencing center. Such an effect is achieved by setting a control upper-limit frequency.